Image reject mixer circuit arrangements

ABSTRACT

An image reject mixer for a radio receiver comprises transconductors  21  and  22 , mixer stages  30  and  23  and a phase shift and combiner circuit  26.  The transconductors  21  and  22  provide differential output current signals to their respective mixer stage  30  and  23 . Capacitors  28  and  29  are connected between equivalent outputs of the transconductors  21  and  22  respectively. The capacitors  28  and  29  have the effect of correlating the output noise of the transconductors  21  and  22  and correlating the noise generated by the mixer stage transistors which is leaked to the inputs of the mixer stages  30  and  23 , the image frequency components of which noise are thereby cancelled by the operation of the mixer stages  30  and  23  and the phase shift and combiner circuit  26.  The capacitors  28  and  29  also compensate the second harmonic of the local oscillators which leak through to the inputs of the mixer stages  30  and  23.  Overall, gain, noise figure and linearity can all be improved without an increase in current consumption.

FIELD OF THE INVENTION

The present invention relates to image reject mixer circuit arrangementsand in particular, although not exclusively, to image reject mixercircuit arrangements for use in radiotelephone RF receiver circuits.

BACKGROUND OF THE INVENTION

There is a continuing drive in radiotelephone receiver design to improvethe linearity characteristics, the power consumption and the noisefigure of the receiver circuitry whilst achieving a suitable level ofreceiver gain. Image reject mixer circuits are commonly used circuitblocks of such receivers. Image reject mixer circuits in which RF inputsignals are arranged to be fed into first and second parallel paths,associated with in-phase and quadrature local oscillator signalsrespectively, and subsequently combined are generally preferred to mixercircuits which have a filter to reject image frequency signals. Thispreference stems from the fact that their noise figure is comparable tothat obtained when an ideal image reject filter is used, which of courseis not possible, and they tend to take up less chip area and/or involvefewer discrete components than mixer circuits having image rejectfilters. Whatever type of image reject mixer circuit is used in a radioreceiver, its parameters determine the main characteristics of thereceiver.

BRIEF SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided an image reject mixer circuit arrangement in which inputsignals are arranged to be fed into first and second parallel paths,associated with in-phase and quadrature local oscillator signalsrespectively, and subsequently combined comprising in each path acurrent signal source circuit arranged to provide from first and secondoutputs thereof differential current signals, dependent on the inputsignals, to a mixer stage characterized in having a capacitor connectedbetween the first outputs of the current signal source circuits of thefirst and second paths.

In accordance with a second aspect of the present invention, there isprovided an image reject mixer circuit arrangement comprising:

an input;

first and second current signal source circuits each having an input andfirst and second outputs;

first and second mixer stages each having first and second signalinputs, a local oscillator signal input and first and second outputs;

a combiner circuit having first to fourth inputs and an output;

a capacitor having first and second electrodes; and

an output;

the input being connected to the first current signal source circuitinput and to the second current signal source circuit input, the firstcurrent signal source first and second outputs being connected to thefirst mixer stage first and second signal inputs respectively, thesecond current signal source first and second outputs being connected tothe second mixer stage first and second signal inputs respectively, thefirst mixer stage first and second outputs being connected to thecombiner circuit first and second inputs respectively, the second mixerstage first and second outputs being connected to the combiner circuitthird and fourth inputs respectively, the combiner circuit output beingconnected to the output, the capacitor first electrode being connectedto the first current signal source circuit first output and thecapacitor second electrode being connected to the second current signalsource circuit first output.

The current signal source circuits are preferably transconductors butmay alternatively be current amplifiers, phase splitters or the like.The primary requirement is that they provide differential currentsignals dependent on the input signals to their respective mixer stage.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample only with reference to the accompanying drawings, of which:

FIG. 1 shows a prior art image reject mixer circuit arrangement;

FIG. 2 shows a typical commutation stage, mixer core or mixer stage;

FIG. 3 shows a second prior art image reject mixer circuit arrangement,used to explain noise present in and generated by components within animage reject mixer; and

FIG. 4 shows an image reject mixer circuit arrangement in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a popular Gilbert cell based image reject mixer circuitarrangement. The mixer circuit arrangement comprises an input terminal 1which is connected to signal inputs of both of a first transconductor 2and a second transconductor 3 by a node 1A. The first transconductor 2has first and second outputs which are connected to first and secondsignal inputs respectively of a first mixer stage 4 by respectiveconnections 2A and 2B. The second transconductor 3 has first and secondoutputs which are connected to first and second signal inputsrespectively of a second mixer stage 5 by respective connections 3A and3B. The first mixer stage 4 is arranged to receive in-phase localoscillator signals at local oscillator input terminals 7. The secondmixer stage 5 is arranged to receive quadrature local oscillator signalsat local oscillator input terminals 8. Differential output currentsignals from the first and second mixer stages 4 and 5 are provided torespective ones of first to fourth inputs of a phase shift and combiningcircuit 6, where they are combined. The combined signals are applied tooutput terminals 9.

The mixer stages 4 and 5 may each comprise the mixer stage of FIG. 2,although other mixer stages may also be used.

An analysis of the FIG. 4 image reject mixer circuit arrangementprovides basic noise properties of such systems, from whichmodifications for optimising the circuit arrangement can be derived.

In FIG. 3, an RF input signal applied to a terminal 10 is received andamplified by a low noise amplifier (LNA) 11, a node B at the output ofwhich is connected to signal inputs of both of an in-phase mixer stage12 and a quadrature mixer stage 13. Local oscillator signals provided ata terminal 14 are applied directly to a local oscillator input of thequadrature mixer stage 13 and, by way of a 90° phase shifter 15, to alocal oscillator input of the in-phase mixer stage 12. The outputs ofthe in-phase mixer stage 12 and the quadrature mixer stage 13 areconnected by way of nodes C and D respectively to respective inputs of aphase shift and combining circuit 16, the output of which is connectedby a node E to an output terminal 17. The operation of this image rejectmixer circuit will be understood by the person skilled in the art.

Assuming that the phase shift and combiner circuit 16 is noiseless, thenoise present at points throughout the circuit can be calculated asfollows:

The noise power at node B is:

N _(LNAt) =N _(S) A _(P) +N′ _(S) A′ _(P) +N _(LNA) +N′ _(LNA)  (1)

where . . .

N_(S), N′_(S)=Source noise at the wanted and image frequencies,respectively;

A_(P), A′_(P)=LNA power gain at the wanted and image frequencies,respectively; and

N_(LNA), N′_(LNA)=Noise due to the LNA circuit at the wanted and imagefrequencies respectively.

The noise power at the outputs of each of the in-phase and quadraturemixer stages 12 and 13 (nodes C and D) is . . .

N _(PQ)=½(N _(S) A _(P) +N _(LNA))G _(P) +N _(M)+½(N′ _(S) A′ _(P) +N′_(LNA))G′ _(P) +N′ _(M)  (2)

where . . .

N_(M), N′_(M)=Noise due to the mixer stages 12 and 13 at wanted andimage frequencies, respectively; and

G_(P), G′_(P)=Power gain at the mixer stages 12 and 13 respectively.

It should be noted that the factor of ½ in equation (2) reflects thefact that the signal power at the output of the LNA is equally dividedbetween the in-phase mixer stage 12 and the quadrature mixer stage 13.

Noise from the source and from the LNA 11 at the output of the in-phasemixer stage 12 will be correlated with noise from the source and fromthe LNA 11 at the output of the quadrature mixer stage 13. However,noise generated in the mixer stages 12 and 13 will not be correlated.Therefore, when the signals from the in-phase and quadrature mixerstages 12 and 13 are summed, the noise currents from the source and LNA11 at the signal frequency are summed, noise from the source and fromthe LNA 11 at the image frequency are rejected, and noise power from thein-phase mixer stage 12 and the quadrature mixer stage 13 is summed.Therefore, after summation (at node E), the total output noise power is. . .

 N _(tot)=2N _(s) A _(P) G _(P)+2N _(LNA) G _(P)+2(N _(M) +N′ _(M))  (3)

Hence the noise figure . . . $\begin{matrix}\begin{matrix}{{NF} = {\frac{N_{tot}}{2N_{s}A_{P}G_{P}} = {1 + \frac{N_{LNA}}{N_{S}A_{P}} + {\frac{1}{A_{P}}( \frac{N_{M} + N_{M}^{\prime}}{N_{S}G_{P}} )}}}} \\{= {{NF}_{LNA} + {\frac{1}{A_{P}}( {{NF}_{SSB} - 1} )}}}\end{matrix} & (4) \\{{where}\quad \ldots} & \quad \\{{NF}_{LNA} = {1 + \frac{N_{LNA}}{N_{S}A_{P}}}} & (5) \\{{NF}_{SSB} = {1 + \frac{N_{M} + N_{M}^{\prime}}{N_{S}G_{P}}}} & \quad\end{matrix}$

It should be noted that the final expression above (4), is the sameresult as would be obtained for a conventional system of the sameoverall conversion gain and linearity performance with an ideal imagefrequency signals rejecting filter between the LNA and mixer.

In practice the input transconductor can have common blocks for both Iand Q mixers. In this case the noise generated in the common block iscorrelated and hence the image part of the noise will be rejected. Thetheory suggests therefore that an image reject mixer can achieve abetter noise figure if the input transconductor is common for both mixerstages.

It is more convenient however to use a separate transconductor for eachmixer stage. Such an arrangement does not suffer from problems caused byDC mismatch, which is inevitable when mixer stages are DC connected to asingle transconductor. Even a small DC mismatch between the inputs ofthe mixer stages will result in unbalanced DC currents in these stages.This will result in leakage of RF and local oscillator frequency signalsto the mixer output and will also increase noise flow from the localoscillator path.

In FIG. 4, in an image reject mixer circuit arrangement in accordancewith the present invention, an input terminal 20 is connected to signalinputs of both of a first transconductor 21 and a second transconductor22 by a node F. The first transconductor 21 has first and second outputswhich are connected to first and second signal inputs respectively of afirst mixer stage 30 by a respective one of first and second nodes G andH. The second transconductor 22 has first and second outputs which areconnected to first and second signal inputs respectively of a secondmixer stage 23 by a respective one of third and fourth nodes I and Jrespectively. The mixer stage 30 is arranged to receive in-phase localoscillator signals at local oscillator input terminals 24. The mixerstage 23 is arranged to receive quadrature local oscillator signals atlocal oscillator input terminals 25. Differential output current signalsfrom the first and second mixer stages 30 and 23 are provided torespective ones of first to fourth inputs of a phase shift and combiningcircuit 26, where they are combined. The combined signal is applied tooutput terminals 27. A first capacitor 28 is connected between the nodesG and I, corresponding to the first outputs of the first and secondtransconductors 21 and 22, and a second capacitor 29 is connectedbetween nodes H and J, corresponding to the second outputs of the firstand second transconductors 21 and 22. The first outputs of the first andsecond transconductors 21 and 22 are equivalent to each other, as arethe second outputs.

Each of the transconductors 21 and 22 converts the RF single endedvoltage signal received at the terminal 20 and provides in dependencethereon differential output current signals on its respective first andsecond outputs. The current signal provided on the first output of a oneof the first and second transconductors is in anti-phase to the signalprovided on the second output of that transconductor. Also, because thetransconductors 21 and 22 receive the same input signal and have thesame internal configuration, the signals provided on their respectiveoutputs are approximately the same as those provided on the equivalentoutput of the opposite transconductor 21 and 22. The operation of themixer stages 30 and 23 and the phase shift and combiner circuit 26 isconventional.

The connection of the capacitors 28 and 29 between the first outputs andthe second outputs respectively of the transconductors 21 and 22 has anumber of positive effects on the operation of the image reject mixercircuit, as explained below:

The capacitors 28 and 29 cause the output noise of the transconductors21 and 22 to become correlated. They also cause the noise generated atthe signal inputs of the mixer stages 30 and 23 by the transistorswithin them to become correlated. The components of the noise from bothof these sources which are at the image frequency will thus be rejectedby the operation of the mixer stages 30 and 23 and the phase shift andcombining circuit 26. Also, the power of the second harmonics of thelocal oscillator frequency that is leaked through to the signal inputsof the mixer stages 30 and 23 is significantly reduced. This is becausethe second harmonics produced by the in-phase and quadrature localoscillator signals in the mixer stages 30 and 23 appear at the signalinputs of the mixer stages 30 and 23 respectively 180° out of phase. Thecapacitors 28 and 29 compensate the second local oscillator harmonicsignals which appear across them to reduce their amplitude. Inconventional image reject mixer circuits, the second harmonic of thelocal oscillator frequency disturbs significantly the switching of thetransistors within the mixer stages and leads to gain degeneration,especially at high frequencies.

The inclusion of the capacitors 28 and 29 thus result in an improvementin the overall conversion gain, the noise figure and the third orderintercept point of the image reject mixer circuit arrangement. Theextent of these improvements increases as the local oscillator frequencyincreases. For radio receivers operating at 900 or 1800 MHz, optimumperformance will be achieved by the inclusion of capacitors 28 and 29having a value in the range of 0.5 to 4.0 pF. However, the value ofcapacitors 28 and 29 needed to provide optimum benefit will dependparticularly on the frequency of the RF input signal and the particularsof the image reject mixer circuit's design. The inclusion of thecapacitors 28 and 29 into the image reject mixer circuit arrangementdoes not increase the overall current consumption of the circuit.

What is claimed is:
 1. An image reject mixer circuit arrangement inwhich input signals are arranged to be fed into first and secondparallel paths, associated with in-phase and quadrature local oscillatorsignals respectively, and subsequently combined comprising in each patha current signal source circuit arranged to provide from first andsecond outputs thereof differential current signals, dependent on theinput signals, to a mixer stage characterised in having a capacitorconnected between the first outputs of the current signal sourcecircuits of the first and second paths.
 2. A mixer circuit arrangementin accordance with claim 1 further comprising a second capacitorconnected between the second outputs of the current signal sourcecircuits of the first and second paths.
 3. A mixer circuit arrangementin accordance with claim 1 in which the current signal source circuitsare transconductor circuits.
 4. A radio receiver including an imagereject mixer circuit arrangement in accordance with claim
 1. 5. Aradiotelephone including a radio receiver in accordance with claim
 4. 6.An image reject mixer circuit arrangement comprising: an input; firstand second current signal source circuits each having first and secondoutputs; first and second mixer stages each having first and secondsignal inputs, a local oscillator signal input and first and secondoutputs; a combiner circuit having first to fourth inputs and an output;a capacitor having first and second electrodes; and an output; the inputbeing connected to the first current signal source circuit input and tothe second current signal source circuit input, the first current signalsource first and second outputs being connected to the first mixer stagefirst and second signal inputs respectively, the second current signalsource first and second outputs being connected to the second mixerstage first and second signal inputs respectively, the first mixer stagefirst and second outputs being connected to the combiner circuit firstand second inputs respectively, the second mixer stage first and secondoutputs being connected to the combiner circuit third and fourth inputsrespectively, the combiner circuit output being connected to the output,the capacitor first electrode being connected to the first currentsignal source circuit first output and the capacitor second electrodebeing connected to the second current signal source circuit firstoutput.
 7. An arrangement, in accordance with claim 6, furthercomprising a second capacitor having first and second electrodes, thesecond capacitor first electrode being connected to the first currentsignal source second output and the second capacitor second electrodebeing connected to the second current signal source second output.